Don't know, I'm violating that rule against descendants of OCs who helped in the war because "f*** you, I can ;D" (Also because it's central to the story we're trying to tell and it's the main reason we're trying one at all.)
I'd say use your better judgement, Kkat built a world for us to work with and left a lot of holes for us to either fill or leave. Just always remember that what you put in your story is going to get different reactions from other people, and if you go around stepping everywhere blindly you might find some angry folks with aching toes when you look back on your path.
Fact of the matter is, your characters in any Fallout Equestria sidefic are probably the descendants of OCs who took part in the war, because there aren't enough canon ponies to around, and ostensibly most ponies in Equestria took part in the war to some degree or another, if someone wants to get hung up on it if you try to explain it, that's their problem.